1. Field of the Invention
The present invention relates to stud fasteners, and in particular to expandable anchor studs in which a threaded bolt or threaded rod carries spaced oppositely tapered expansion plugs which, when axially forced against each other, cause the two or more shell segments confined between the expansion plugs to expand radially, thereby providing an anchoring engagement inside a suitable anchoring bore.
2. Description of the Prior Art
From the prior art teachings in this field are known various types of expandable anchor studs, including anchor studs designed for heavy-duty applications, where it is necessary that the segments of the stud shell expand on both axial ends. One such prior art solution, disclosed in the German Printed Application (Offenlegungsschrift) No. 2,046,341, suggests an expanding anchor stud in which the longitudinally split segments of the stud shell are metal shell segments which abut against each other on their longitudinal edges. They are radially held together by means of plastic end rings fitted onto reduced-diameter end portions of the shell segments. This design is primarily intended to facilitate the the insertion of the anchor stud into its bore, while also being aimed at a simplification of its manufacture. However, one shortcoming of this prior art solution relates to the fact that the grooves, or the reduced-diameter end portions of the shell segments, on which the plastic rings are received are arranged in the same longitudinal portions of the segments which are also weakened from the inside by the provision of end tapers with which they cooperate with the expansion plugs on the threaded bolt. This means that the wall thickness of the shell segments is weakened from both sides. In order to minimize the depth of the outer diameter weakening, the plastic rings are therefore kept rather thin. This fact now entails a second shortcoming, namely the premature expansion of the anchor stud, when the threaded bolt is pre-tightened by hand, even if only slightly, prior to insertion, because in most cases the corresponding anchoring bore in the concrete wall or foundation is only some tenths of a millimeter larger than the non-expanded anchor stud. Thus, the overly easy premature expansion of the anchor stud may prevent its full insertion into the anchoring bore.
It was therefore found to be preferable that the shell segments are first expanded on their penetrating end portion. For this purpose, one would first slightly tighten the expansion plug on the penetrating side of the stud, which consequently would sit somewhat deeper inside the shell taper than the opposite expansion plug located near the mouth of the anchoring bore. It can now happen, however, that, during insertion of the anchor stud, the shell segments are axially shifted in relation to the threaded bolt, due to frictional engagement between the slightly expanded shell end and the wall of the anchoring bore, so that the shell segments ride up on the rear expansion plug, with the resulting risk that the anchor stud can not be fully inserted into the bore, because the shifted shell segments present a taper which is increasing in diameter toward the rear of the anchor stud.